MicrobiologyBytes

Recently some of my final year virology students commented to me that it was a shame I was not still making regular MicrobiologyBytes podcasts. But as I said when I stopped posting weekly podcasts, I don’t feel that audio adds value to the content of what I like to share here. But video may be a different matter. Not that you’re going to see videos of me talking, because you won’t learn much microbiology that way, and I’ve only ever seen one good talking head video. But some papers lend themselves to a more visual treatment, and this is a very good example:

Structural dissection of Ebola virus and its assembly determinants using cryo-electron tomography. PNAS USA 27 February 2012 doi: 10.1073/pnas.1120453109
Ebola virus is a highly pathogenic filovirus causing severe hemorrhagic fever with high mortality rates. It assembles heterogenous, filamentous, enveloped virus particles containing a negative-sense, single-stranded RNA genome packaged within a helical nucleocapsid (NC). We have used cryo-electron microscopy and tomography to visualize Ebola virus particles, as well as Ebola virus-like particles, in three dimensions in a near-native state. The NC within the virion forms a left-handed helix with an inner nucleoprotein layer decorated with protruding arms composed of VP24 and VP35. A comparison with the closely related Marburg virus shows that the N-terminal region of nucleoprotein defines the inner diameter of the Ebola virus NC, whereas the RNA genome defines its length. Binding of the nucleoprotein to RNA can assemble a loosely coiled NC-like structure; the loose coil can be condensed by binding of the viral matrix protein VP40 to the C terminus of the nucleoprotein, and rigidified by binding of VP24 and VP35 to alternate copies of the nucleoprotein. Four proteins (NP, VP24, VP35, and VP40) are necessary and sufficient to mediate assembly of an NC with structure, symmetry, variability, and flexibility indistinguishable from that in Ebola virus particles released from infected cells. Together these data provide a structural and architectural description of Ebola virus and define the roles of viral proteins in its structure and assembly.

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Researchers have developed a single vaccine which protects against both rabies and Ebola virus. These two viruses are related to each other, but do not cross-react serologically. By inserting elements of the Ebola virus GP protein into an existing rabies virus vaccine, a single bivalent vaccine was produced. Although it works in the laboratory, the new vaccine – or something similar based on this first attempt – need to be tested in primates and eventually in humans.

Apart from people, Ebola virus is thought to have eradicated thousands of gorillas, prompting the World Conservation Union to raise their status to “critically endangered” in 2007, the first time a mammal has become critically endangered as a direct result of disease. Vaccination could help prevent future deaths.

Inactivated or Live-Attenuated Bivalent Vaccines that Confer Protection against Rabies and Ebola Viruses. J Virol. Aug 17 2011
The search for a safe and efficacious vaccine for Ebola virus continues as no current vaccine candidate is nearing licensure. We have developed (a) replication-competent, (b) replication-deficient, and (c) chemically inactivated rabies virus (RABV) vaccines expressing Zaire ebolavirus (ZEBOV) glycoprotein (GP) using a reverse genetics system based on the SAD B19 RABV wildlife vaccine. ZEBOV GP is efficiently expressed by these vaccine candidates and is incorporated into virions. The vaccine candidates were avirulent after inoculation of adult mice, and viruses with a deletion in the RABV glycoprotein have greatly reduced neurovirulence after intracerebral inoculation in suckling mice. Immunization with live or inactivated RABV vaccines expressing ZEBOV GP induced humoral immunity against each virus and conferred protection from both lethal RABV and EBOV challenge in mice. The bivalent RABV/ZEBOV vaccines described here have several distinct advantages that may speed the development of inactivated vaccines for use in humans and potentially live or inactivated vaccines for endemic nonhuman primates at risk of EBOV infection.

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Ebola and Marburg virus are filoviruses that cause outbreaks of highly lethal haemorrhagic fever. Mortality rates in these diseases average more than 50%, with the highest recorded rates seen for Ebola Zaire virus (88%) and Marburg Angola virus (90%). Infection with these filoviruses produces a very high fever followed by interference with blood coagulation and vascular permeability, causing internal bleeding, bruising and skin rashes. After an asymptomatic incubation period, which can last days to weeks, symptoms of a typical filovirus infection emerge; headache, nausea, fever and malaise followed by more serious haemorrhagic symptoms and, in fatal cases, death results from multi-organ failure owing to shock.

Present treatments for filovirus infection are palliative, and consist primarily of supportive care, including hydration and pain management. There is no effective treatment or cure for these diseases. Therefore, vaccine development is crucially important as a strategy for fighting filovirus outbreaks. However, vaccine efficacy testing for Ebola virus is very difficult. There is no readily identifiable high-risk human population that can be targeted for a placebo-controlled clinical trials because disease outbreaks are unpredictable and sporadic, both geographically and temporally. Normally, clinical trials of medicines and vaccines intended for human use follow a lengthy but predictable sequence of safety and efficacy testing.

Because of its sporadic nature, the incidence of Ebola virus infection in human populations is not predictable and does not allow for adequate testing. Moreover, the immune correlates of protection from filovirus disease in humans remain unknown and therefore cannot be used to assess candidate vaccine efficacy. To facilitate the licensing of medicines when efficacy cannot be evaluated in the setting of natural infection, the U.S. Food and Drug Administration (FDA) introduced a new regulation in 2002 as an alternative licensing pathway for pharmaceutical products that target highly lethal pathogens. The FDA’s “animal rule” allows approval based on animal efficacy data. The animal rule is intended to be used as a pathway for regulatory approval only when there is no other way to licence a vaccine (Correlates of protective immunity for Ebola vaccines: implications for regulatory approval by the animal rule. 2009 Nature Reviews Microbiology 7: 393-400).

In the case of Ebola virus, the relevant animal models are non-human primates and mice. The immune correlates of Ebola virus infection consist of immunoglobulin G responses, although other factors, such as T cells, are also likely to be important in a successful immune response. Current vaccine candidates against Ebola virus include the virus glycoprotein and nucleocapsid proteins. Initial animal testing of Ebola vaccines has shown a protective effect in non-human primates and positive antibody titres in humans.

To date, no vaccines have received regulatory approval and been licensed using the FDA animal rule. This pathway does not diminish the level of regulatory contol required for vaccine approval; extensive human testing is still required to demonstrate safety and immunogenicity. The predictive relationship between animals and humans for protective efficacy is unknown, and therefore an immune correlate is used to bridge the gap between animal efficacy studies and human immunogenicity trials. It has not yet been determined what level of efficacy in animals will be required for vaccine approval, but other vaccines currently administered to the U.S. population have shown efficacies in human trials that are as low as 18%. Even this level of efficacy will provide a benefit against pathogens such as filoviruses with high mortality rates, and therefore may be acceptable against emerging natural infections or bioterrorism threats.

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